Hey! Good job on submitting a game! I've got some feedback for ya.
I see how you fit the theme of "Destroy Everything", but I'm not so sure where the polymorphic part factors into the gameplay, other than speed. Once I realized that the green coins transformed me, I also realized that rats are the fastest, and once I became one, I just never switched again. There was no (apparent) point. There's also a bit of a disconnect given that every form just uses a gun.
Killing 150 enemies seems a bit much, especially given the size of the map, which is HUGE. I got to 119, but after that, I just couldn't find any more baddies. I'm also confused as to why they have guns if they don't shoot at you?
But good job nonetheless! Keep it up!